FFT analysis with 24bit soundcard vs. 18 bit NI-DAQ - diyAudio
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Old 7th July 2008, 03:15 AM   #1
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Default FFT analysis with 24bit soundcard vs. 18 bit NI-DAQ

Hello,

I am currently using an 18-bit NI-DAQ ADC / DAC card (PCI-6281) to perform some sensitive tone level and distortion measurements over time via FFTs. As a preamp, I am using the LT1678 which has exceptional distortion and noise specs, particular at low frequencies - the 1/f corner frequency is 5Hz or so. I also tried the same measurement with a comparable 16-bit NI-DAQ card (PCI-6221), and found that the noise in my measurements did increase.

So now that it appears that bit resolution matters in my measurements, I am thinking about 24bit acquisition. National Instruments has some very expensive cards, too expensive to justify the purchase if I just want to try it. But maybe one of the many 24bit sound cards could be worth a try. Especially the EMU 1212M is affordable and has a low noise floor.

So my question is, has someone already tried and compared NI-DAQ cards to good sound cards for tone analysis? E.g., measuring the amplitude stability of a 1kHz tone over time?

I did read a related post in this forum by tschrama here, which claims that even a 16bit NI-DAQ card is better than a 24bit sound card, but I have a few questions for him that I can't ask him directly yet as a new member. For example, what software did he use to do this comparison? LabView default interfaces are limited to only 16bit when it comes to soundcards (probably on purpose). But I also know that you can get some improved interfaces over the web, for example here, which allow you to acquire up to 32bit into LabView over your soundcard.

So does someone have experience with using soundcards and labview to do spectral analysis work? Any help is greatly appreciated.
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Old 7th July 2008, 09:01 AM   #2
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Dear Sebastian,

Indeed these test were done some years ago now.. so maybe tings have imrpoved.

The PCI-6281 looks like an very good DAQ card. I've been hunting one of these for some time now on ebay, but there are still above my budget. I doubt that you will improve on the PCI-6281 with a sound card. Maybe the EMU 1212M is an eceptional good sound card and it will be better, but the Audigy ZS won't. I think I once looked up the specs of the opamps used on the Audigy ZS card, and those were cheap noise opamps.

best to check with the EMU 1212M manufacturer and read the specifications very good. But NO soundcard is going to give you 24bit real bits of resolution, maybe they come close to 18bits.

My bet would be on the PCI-6281!

Kindest regards,
Thijs
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Old 7th July 2008, 09:34 AM   #3
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Hi Sebastian,

A few years ago, I've tested the EMU 1212M sound card. As far as I can remember, the DAC and ADC were okay, but the rest was lousy: cheap op-amps etc.
I had the same problem with the "Waveterminal 192X" from ESI, and replaced most of the op-amps, caps and resistors by better ones. It was a terrible job, once is enough.

A really good sound card is the Lynx L22, regrettably quite expensive.

Regards,
Edmond.

PS: As for software, I've no experience with Labview. Can't help, sorry. I'm using my own software.
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Old 7th July 2008, 12:43 PM   #4
Gordy is offline Gordy  United Kingdom
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I don't want to take the thread off-course, but if you do go for the E-Mu card then watch out for PCI compatibility... it does not work with the latest iteration... needs PCI version 1 or 2 if I remember correctly.
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Old 7th July 2008, 06:40 PM   #5
weissi is offline weissi  Europe
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If you don't need the balanced I/O you can try to grab an used EMU 0404 too, the quality of the converters isn't really worse. But you will not come around in building better analog stages around the converters. There are the expensive things that make the difference.
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Old 7th July 2008, 07:13 PM   #6
fotios is offline fotios  Greece
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If the following graphs help to solve your question, these are the IMD measurements of an EMU-0404 with the use of the software Multi-Instrument Pro.3. Both the M.I. Pro.3 and the EMU-0404 are in position to run in 198Ks/s at 24bit, according to the possibilities of P.C. The P.C. that gave these graphs was an Intel C2D/3GHz with DDRAM 8GB and OS WinXP x64. Any other P.C. it hung-ups under such high sampling rate and resolution. The concrete P.C. it needs about 10sec!! to complette each one of measurements and the CPU usage touches during each process about 80%!!

CCIF standard meas.
Click the image to open in full size.

DIN standard meas
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SMPTE standard meas
Click the image to open in full size.

Fotios
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